Bhagavad-Gita The Song Divine Chapter 18 (stanza 36-47)

Now hear from Me the threefold joy too. That in which the striver finds enjoyment through practice of adoration, meditation and service to God etc., and whereby he reaches the end of sorrow - such a joy, though appearing as poison in the beginning, tastes like nectar in the end; hence that joy, born as it is of the placidity of mind brought about by meditation on God, has been declared as Sattvika.  ……………. (36-37)

The delight which ensues from the contact of the senses with their objects is eventually poison- like, though appearing at first as nectar; hence it has been spoken of as Rajasika. ………….. (38)

That which stupefies the Self during its enjoyment as well as in the end - derived from sleep, indolence and obstinate error, such delight has been called Tamasika. ……….. (39)

There is no being on earth, or in the middle region or even among the gods or anywhere else who is free from these three Gunas, born of Prakrti.  ……….. (40)

The duties of the Brahmanas, the Ksatriyas and the Vaisyas, as well as of the Sudras have been assigned according to their innate modes of Prakrti (Gunas), Arjun. ……….. (41)

Subjugation of the mid and senses, enduring hardships for the discharge of one's sacred obligations, external and internal purity, forgiving the faults of others, straightness of mind, senses and behavior, belief in the Vedas and other scriptures, God and life after death etc., study and teaching of the Vedas and other scriptures and realization of the truth relating to God - all these constitute the natural duties of a Bhahmana. …………. (42)

Heroism, majesty, firmness, diligence and dauntlessness in battle, bestowing gifts, and lordliness - all these constitutes the natural duty of a Ksatriya. ……………. (43)

Agriculture, rearing of cows and honest exchange of merchandise - these constitute the natural duty of a Vaisya (a member of the trading class); and service of the other classes is the natural duty even of a Sudra (a member of the laboring class) ……. (44)

Keenly devoted to his own natural duty, man attains the highest perfection in the form of God-realization. Hear the mode of performance whereby the man engaged in his inborn duty reaches that highest consummation.  ……………… (45)

From whom all beings come into being and by whom the whole universe is pervaded, by worshipping Him through the performance of his own natural duties, man attains the highest perfection.  …………….. (46)

Better is one's own duty, though devoid of merit, than the duty of another well-performed; for, performing the duty ordained by his own nature, man does not incur sin.  ….. (47)